What is a synonym for couldve?

"Could've" is a contraction of "could have". Here are some synonyms depending on the context:

Formal:

* Might have

* May have

* Was able to

* Had the possibility to

* Had the capacity to

Informal:

* Maybe

* Perhaps

* Possibly

* Could have been

* Would have been able to

The best synonym will depend on the specific sentence you are trying to write.